hotplug ownership/permissions

[Scott Cuyle Fritzinger <scottf@med.unr.edu>]

Playing around with hotplug and noticed that it will set the owner of the
/proc/bus/usb/xxx/yyy device to the locally logged in user. Is this normal
behavior, or have I done something weird? This is one thing I wanted the
script to do, but don't know if this has been the behavior all along, or
if I did something weird. :P

Also, i'm starting to hack around with usb.agent on my machine to do the
following:

1) set ownership/permissions on the device to locally logged in user (but
this looks like it's already done ;)

2) split the usb.handmap out into multiple files located in
/etc/hotplug/usb.maps . the usb.agent will look in all files in the dir.

3) for EVERY match on the device, load the module, or run the associated
script (located in /etc/hotplug/usb.scripts). 

here's the logic behind this: by running every matching map entry's
module/script, we can guarantee that a single script won't hog the device.
the usb.agent will set owner/perms AFTER every script has run so that the
perms are guaranteed.

Consider this scenario: 

- a PTP camera is connected. 
- there are 2 map matching map entries. one in each:
	/etc/hotplug/usb.maps/jphoto
	/etc/hotplug/usb.maps/gphoto
  the least specific matching map entry (class, then vendor/product id) is
  run first

- the jphoto map entry calls the /etc/hotplug/usb.scripts/jphoto to dump
  all pictures to disk (check David's great hotsync script at
  jphoto.sourceforge.net)
- the gphoto map entry calls /etc/hotplug/usb.scripts/gphoto to maybe do
  the same, or to signal the desktop that a camera has been plugged in.
- the usb.agent sets owner/permissions to the locally logged in user, or
  root if none.

Or this one:

- a mass-storage camera is connected.
- there are 2 map entries:
	/etc/hotplug/usb.maps/mass-storage   (matches storage class)
	/etc/hotplug/usb.maps/gphoto-storage (matches the VID/PID)

- the mass-storage entry loads the SCSI emulation and other kernel modules
  necessary for proper operation. (not necessary?)
- the gphoto entry calls /etc/hotplug/usb.scripts/gphoto-storage that will
  then mount the camera to /camera0 and set the perms on the mountpoint to 
  local user (passed via env variable $USER)
- the usb.agent sets owner/permissions to the locally logged in user, or
  root if none.

i know there were mentions of behaving properly with the modules.usbmap
file, so the mass-storage map above is probably not necessary as the
kernel will load the class driver automatically. so ignore that if it
isn't needed :)

It has the following benefits:

- Multiple applications can do whatever they want (jphoto, gphoto, etc..)
if they place a map in /etc/hotplug/usb.maps/   not one application 'hogs'
the device.

- this allows for a nice expansion of usb capabilities. Apps can know
right away if a device has been plugged, unplugged, etc... Think about a
KDE script that notifies the desktop a camera has been plugged in and it
opens konqueror automatically, or puts an icon on the desktop right away.
same for GNOME/Nautilus... or whatever!

- local user has perms on the device. no more emails from people about not
being able to access their digicams (OK OK, this is mainly for gphoto's
benefit ;)... other devices as well though...

- !!EASY!! to add and remove map entries. add or remove a file at
application installation/removal. no more sed'ing through the
usb.{user|hand}map files.

Here's a quick question now: Is the hotplug architecture serialized? are
we guaranteed that only 1 device will be processed by hotplug at a time?

What do people think of this?

Thanks for listening!

-=Scott Fritzinger


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